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AC and DC generators; alternating and direct current compared sheet
5-mark answer frame

Name the main parts of an AC generator and explain how it works. Draw a labelled diagram.

How to lay out the answer. Follow the frame, then write it in your own words.

  1. 1 · State the principle. An AC generator works on electromagnetic induction: a coil turning in a magnetic field has a current induced in it.
  2. 2 · Name the parts.
    • A strong magnet with N and S poles
    • A rectangular armature coil ABCD of many turns
    • Two slip rings and two carbon brushes
    • An axle turned by a turbine or engine
  3. 3 · Draw the diagram. [Sketch: magnet poles N and S, coil ABCD between them, slip rings on the axle, brushes joined to a galvanometer; all labelled]
  4. 4 · First half turn. AB moves down and CD moves up through the field. A current is induced in both, and they add up round the coil.
  5. 5 · Second half turn. AB and CD swap places, so each moves the other way and the current reverses.
  6. 6 · The output. Slip rings pass this reversing current to the outer circuit, so it is AC, changing direction twice each turn.
  7. 7 · Link to daily life. Power stations use huge AC generators; in India the supply is 50 Hz.

Scoring tip: Draw the slip rings as two full rings on the axle. Drawing a split ring here is the commonest error and turns your AC generator into a DC one.